Why did the Spartans have two kings?

Sparta ran on a system of democracy, therefore it had two kings who shared responsibilities. It is evident from Plutarch's; "Life of Lycurgus", that a lawgiver known by the name of Lycurgus derived the Spartan laws from the oracle at Delphi, which outlined how Sparta was to function. Within this Delphic Oracle it was stated that Sparta was to have two kings.

Careful there....Ancient Sparta had no such democracy! Sparta was a dinarchy, under the rule of two consecutive kings along with nine 'ephors' and a council. The common people had no say.

Clarification....

Sparta had two kings. The kings belonged to the Council of Elders (Gerousia) which had 5 Ephors along with nobles who were over 60 years of age. The Spartan government also consisted of an assembly made up of male citizens who were over the age of 30. The assembly itself had little power and was there primarily to support the Kings and Ephors. This was not a democratic government like Athens, but rather a government run by a small elite few.

Two kings, from two families, Agiad and Eurypontids, who descended from Herakles. One would go to war and one would remain in the city.